Cable television systems (CATV) were initially deployed so that remotely located communities were allowed to place a receiver on a hilltop and to use coaxial cable and amplifiers to distribute received signals down to the town that otherwise had poor signal reception. These early systems brought the signal down from the antennas to a “head end” and then distributed the signals out from this point. Since the purpose was to distribute television channels throughout a community, the systems were designed to be one-way and did not have the capability to take information back from subscribers to the head end.
Over time, it was realized that the basic system infrastructure could be made to operate two-way with the addition of some new components. Two-way CATV was used for many years to carry back some locally generated video programming to the head end where it could be up-converted to a carrier frequency compatible with the normal television channels.
Definitions for CATV systems today call the normal broadcast direction from the head end to the subscribers the “forward path” and the direction from the subscribers back to the head end the “return path.” A good review of much of today's existing return path technology is contained in the book entitled Return Systems for Hybrid Fiber Coax Cable TV Networks by Donald Raskin and Dean Stoneback, hereby incorporated by reference as background information.
One innovation, which has become pervasive throughout the CATV industry over the past decade, is the introduction of fiber optics technology. Optical links have been used to break up the original tree and branch architecture of most CATV systems and to replace that with an architecture labeled Hybrid Fiber/Coax (HFC). In this approach, optical fibers connect the head end of the system to neighborhood nodes, and coaxial cables are used to connect the neighborhood nodes to homes, businesses and the like in a small geographical area.
FIG. 1 shows the architecture of a HFC cable television system. Television programming and data from external sources are sent to the customers over the “forward path.” Television signals and data are sent from a head end 10 to multiple hubs 12 over optical links 11. At each hub 12, data is sent to multiple nodes 14 over optical links 13. At each node 14, the optical signals are converted to electrical signals and sent to customers over a coaxial cable 15 in the frequency range of 55 to 850 MHz.
Data or television programming from the customer to external destinations, also known as return signals or return data, are sent over the “return path.” From the customer to the node, return signals are sent over the coaxial cable 15 in the frequency range of 5 to 42 MHz in the U.S. At the nodes 14, the return signals are converted to optical signals and sent to the hub 12. The hub combines signals from multiple nodes 14 and sends the combined signals to the head end 10.
FIG. 2 is a block diagram of the return path portions of a conventional HFC system. Analog return signals are routed over coaxial cables 15 to nodes 14 where the signals are converted by analog to digital (A/D) converters 142 to digital return signals. The digital return signals are transmitted over digital optical links to a hub 12. The sample rate of each digital return signal is determined independently. A typical sample rate is approximately 100 MHz, but it varies from node to node.
The hub 12 receives digital return signals from two or more (any typically many) nodes 14. In a typical system there are fewer fiber links to the head end than there are fiber links coming from the nodes. Accordingly, the signals from multiple nodes must be combined. But these digital return signals, which are asynchronous, cannot be summed directly. Thus, the digital return signals are converted to analog format by Digital Receivers with Analog Outputs 122. Then, the analog return signals are summed by an Analog Summer 124. The summed signal is converted back to a combined digital return signal by AID converter 126 for transmission to the head end 10 via an optical link.
A more detailed block diagram of a conventional hub 12 is illustrated by FIG. 3. As shown, the hub 12 receives digital return signals from multiple nodes 14. Each of the digital return signals is converted into an analog signal by a respective digital-to-analog (D/A) converter 132. Because each digital return signal can have a different sample rate, the hub 12 includes a separate clock recovery circuit 130 for each input signal to generate a recovered clock signal that is used to drive the D/A converter 132. The output of the D/A converter 132 is filtered by an analog filter 134 to remove aliasing artifacts. The Analog Summer 124 then sums multiple signals. After summation the summed signal is converted back to digital signals by an A/D converter 126 for transport to the head end. The A/D converter 126 is driven by an output sample clock, operating at an output sample rate that is typically close to but not identical to the sample rates of the digital signals received from the nodes 14.
The conversion from digital signals to analog signals and back to digital signals is an indirect and contrived process. High-speed digital-to-analog converters and analog-to-digital converters are expensive and add significant cost bases to the HFC equipment. Furthermore, precision errors are inevitably introduced during the conversions.
Accordingly, what is needed is a device for and a method of combining multiple digital return signals in a CATV return path without requiring multiple analog/digital conversions.